I'm pleased to report that I haven't had to turn on the heat yet, though the average temperature these days is only around 50 F. As in the spring, the attached greenhouse on the south side works very well to heat up the house on sunny days, if I remember to open the door. One thing about thermally massive construction is that it does more good if you can let the inside temperature vary somewhat, warm it up during the day so that it doesn't get so cold at night. By letting it vary between about 62 and 78 I've been able to avoid turning on the heat.
At the current outside temperatures it would take about three cloudy days for the inside temperature to drop into the 50's. It helps that the cloudy days tend not to be as cold as the clear days.
I don't know how much longer I can hold out. November tends to be cold and quite cloudy around here.